1. Field of the Invention
This invention relates to a control circuit for automatically regulating a choke valve in a carburetor for an internal combustion engine, and more particularly, to a control circuit for automatically regulating a bimetal actuated choke valve adapted to be driven by heat from an electric heater.
2. Description of the Prior Art
The prior art automatic choke valve cooperates with a bimetal which is driven by heat from an electrical heater responsive to the ambient temperature. Accordingly, the choke valve is regulated for its open and closed positions by varying the amount of electric power to be supplied to the heater in response to the ambient temperature. To this end, the prior art has proposed the use of a control circuit whereby the duty ratio of electric power is automatically changed. Because of the control circuit being provided in a choke housing incorporated in a carburetor, it is necessary to ensure reliability against changes in temperature and humidity, vibrations, dust and dirt, and all other external circumstances that affect the control circuit. Moreover, such a control circuit should be simple to build and operate because of requirements for ease of handling and for minimizing the number of parts to be assembled.
For example, the prior art, in trying to meet these requirements, has employed a hybrid integrated circuit, which integrates therein all the electronic components, such as constant-voltage circuit, temperature-voltage convertor circuit, oscillator circuit, comparator circuit and switching circuit. Because of this configuration, especially of the temperature-voltage convertor circuit incorporated in the hybrid integrated circuit, the use of the hybrid integrated circuit requires due consideration of the characteristics which vary with the type of choke valves or carburetors. When it is desired to finely adjust the variation characteristics of duty ratio with respect to the ambient temperature, the associated regulating resistance effective to determine the temperature-duty ratio characteristic must be trimmed for adjustment by using a solid laser such as YAG laser. This may lead to increased cost of the overall circuit.
Furthermore, this method of laser-trimming is disadvantageous, because the adjustment of resistance value is limited, and the temperature-duty ratio characteristics may not be adjusted over a wide range.
An object of the present invention is to overcome the aforementioned disadvantages of the prior art.
Another object is to provide a control circuit which is extremely simple to operate without modifying resistances provided in the circuit, and yet effective to regulate the temperature-duty ratio characteristics of the circuit.
A further object is to provide a hybrid integrated circuit which is inexpensive to build, compared with conventional hybrid integrated control circuit wherein resistance is controlled by the method of laser-trimming.
A still further object is to provide a control circuit which may be controlled over a fairly wide range.
It is a specific feature of the present invention that the heater controlling circuit for an automatic choke valve is formed by a hybrid integrated circuit to appropriately control temperature-duty ratio characteristics of the hybrid integrated circuit by means of the external resistances RA and RB, thereby enabling adaptation of the identical control circuit for a control circuit for any automatic choke valves in various types of automobiles which are delicately different from one another, depending upon their engine performances with respect to their control characteristics of the automatic choke valves.